Trump made the threat as a partial government shutdown enters its seventh day with no end in sight.

The shutdown began on Saturday after Democrats rejected demands from Trump that $5 billion be included for the wall in a measure to keep the government open.

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"We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with," the president tweeted.

He also criticized past presidents and Congresses over the nation's current immigration laws.

"Hard to believe there was a Congress & President who would approve!"

We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with. Hard to believe there was a Congress & President who would approve!

The tweet was one of several Trump wrote on Friday morning as the shutdown increasingly looked like it could drag well into 2019.

Trump in a second tweet tied the fight over the wall to another of his signature issues: trade. He said the U.S. was losing money through the North American Free Trade Agreement (NAFTA) and that closing the border would be a "profit making operation."

....The United States looses soooo much money on Trade with Mexico under NAFTA, over 75 Billion Dollars a year (not including Drug Money which would be many times that amount), that I would consider closing the Southern Border a “profit making operation.” We build a Wall or.....

The U.S runs a trade deficit with Mexico, but outside observers have said that closing the border could cause economic chaos given the amount of trade and people who legally cross the border every day.

According to a report earlier this month by CBS News, about $558 billion in goods flowed across the border in both directions last year. It found that the U.S. exported $243.3 billion in goods across the border along with $58 billion in services.

Nearly a half million people enter the US. each day at various entry points on the southern border, according to a Wilson Center report cited by CBS.

Duncan Wood, director of the Wilson Center's Mexico Institute, told CBS MoneyWatch that shutting down the border would cost "hundreds of millions of dollars a day" or "maybe a billion."

Trump also said that closing the border would help the U.S. auto industry.

.....close the Southern Border. Bring our car industry back into the United States where it belongs. Go back to pre-NAFTA, before so many of our companies and jobs were so foolishly sent to Mexico. Either we build (finish) the Wall or we close the Border......

He concluded his string of tweets by accusing “Honduras, Guatemala and El Salvador” of taking money from the U.S. and threatening to cut off aid to the countries.

.....Honduras, Guatemala and El Salvador are doing nothing for the United States but taking our money. Word is that a new Caravan is forming in Honduras and they are doing nothing about it. We will be cutting off all aid to these 3 countries - taking advantage of U.S. for years!

Congress appropriates money for foreign aid and would have to sign off on reducing aid to the countries through the appropriations process, something unlikely to happen with Democrats retaking the House majority in January.

Trump negotiated changes to the NAFTA trade deal with Canada and Mexico, but he must get the Congress to agree to legislation implementing the new trade rules to enact them. Democrats are demanding changes to the new deal that would strengthen labor rights provisions.

On the shutdown, Trump and Democrats in Congress have been battling over who is to blame, with the White House stepping up its efforts to shift blame to Democrats.

Democrats are confident that Trump will get the blame from voters because of an Oval Office meeting earlier this month in which Trump said he would embrace a government shutdown that was triggered over the wall.

He also said he would not blame Democrats for a shutdown that started over wall funding, though he has since shifted from that stance.

The shutdown has left 800,000 federal employees furloughed or forced to work without pay through the holidays.

A Reuters/Ipsos poll released Thursday found more people blame Trump for the shutdown than Democrats. In the poll, 33 percent blamed Democrats in Congress for the shutdown, compared to 47 percent who blamed Trump.